1. Field of the Invention
The present invention relates to a high purification method of jig for semiconductor heat treatment. More specifically, it relates to a method of preventing contamination by heat treatment jigs during the heat treatment of semiconductor wafers such as silicon single-crystal wafers.
2. Description of Related Art
Semiconductor device fabrication process includes a number of heat treatment processes, such as oxidation, diffusion and film growth. Hence, a semiconductor wafer incurs various heat treatment in these processes. A variety of jigs are used in semiconductor heat treatment, depending on such factors such as the mode of heat treatment and the type of heating means employed.
For example, in a semiconductor wafer heat treatment process carried out using a vertical heat treatment furnace, a plurality of semiconductor wafers such as silicon single-crystal wafers are loaded and held in a vertical, multi-step wafer holder known as a vertical wafer boat. Such vertical wafer boats are generally made of a material such as quartz glass, single-crystal silicon, polycrystalline silicon, or silicon-impregnated silicon carbide. Recently, to achieve a high level of purity in heat treatment jigs, use has come to be made of heat treatment jigs obtained by employing a chemical vapor deposition (CVD) process (vapor phase growth process) to cover the surface of heat treatment jigs made of the foregoing materials, as the substrate with a silicon carbide (SiC) film, or of heat treatment jigs composed entirely of silicon carbide films grown by chemical vapor deposition as the substrate.
Even in the case of a thin-film vapor-phase growth process onto a wafer surface using an epitaxial growth reactor or the like, a susceptor composed of a graphite substrate that has been SiC coated on the surface by chemical vapor deposition is used as a susceptor for loading the semiconductor wafer, thereby achieving a high level of purity.
Even in heat treatment jigs covered with a SiC film by chemical vapor deposition, it has recently been reported that if the substrate contains a large amount of impurities, during growth of the CVD-SIC film on the surface of the substrate, impurities within the substrate diffuse in a high concentration to the surface of the SiC film, where they remain present. When heat treatment is carried out using such a jig, contamination of the semiconductor wafers occurs (see, for example, Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2000-119079
Patent Document 1 thus discloses a process for manufacturing Si—SiC components for use in semiconductor heat treatment, which process includes a mixing step in which SiC powder containing metal impurities in amounts of not more than 0.05 ppm of iron and not more than 0.05 ppm of nickel, copper, sodium, calcium, chromium and potassium combined is mixed with a binder (or a forming aid), a forming step in which these mixed starting materials are shaped into a green body, a calcination step in which the green body is calcined, a purification step in which the calcined body is purified, an impregnation step in which silicon is impregnated into the purified body, and a processing step in which specific portions on the resulting silicon-impregnated component is rendered to a surface roughness (Ra) of not more than 0.20 μm. Here, “specific portions” refers to places where, when a semiconductor is loaded onto the silicon-impregnated component, the component comes into contact with the semiconductor. This process in Patent Document 1 can provide high-purity jigs for heat treatment which are free of impurity contamination even when not coated with a SiC film or in which, even when coated with a SiC film, impurities do not diffuse into the SiC film.
However, achieving a high level of purity in a substrate material such as this requires processes of a considerable complexity, which invites an increase in production costs and is thus impractical. Moreover, there is a limit to the level of purity that can be increased. For example, even if a high-purity substrate is used and a SiC film is grown on the surface thereof, the SiC film is often contaminated by impurities in the course of SiC film growth.